Homogenous pyrolyses

Hydrocarbons and hydrogen halides are omitted since they will be dealt with elsewhere.) The chemical properties of most of these hydrides are rather well known, but this cannot be asserted for their decomposition kinetics. Some of them are very stable (H20, HF, NH3) while others decompose easily at room temperature (TeH2, PbH4). A study of the homogeneous decomposition has only been undertaken for those elements inside the frame in the Table. The pyrolyses of the others have either been found to proceed heterogeneously or the kinetics is unknown. 

Heterogeneous or surface effects have been found to complicate the interpretation of kinetic experiments, which lead to erroneous Arrhenius parameters. However, with special precautions involving the use of seasoned vessels and the presence of a free-radical suppressor, the errors are minimized. Consequently, the present chapter will cover mostly homogeneous gas-phase processes. Studies on chemical activation, the use of catalysts, the bimolecular gas phase and heterogeneous reactions are not included. As an attempt to describe important pyrolyses data from 1972 to 1992, this review does not pretend to offer a complete coverage of the literature. 

A comprehensive picture of alkyl iodides pyrolyses has been presented by Benson124. These substrates are sensitive and difficult to handle in homogeneous molecular HI elimination studies and this is the reason for the comparatively few gas-phase investigations. Concurrent radical and unimolecular mechanisms are frequently observed in organic iodides decomposition. 

The pyrolyses of benzylamine, iV-methylbenzylamine, and iV-methylaniline have been examined by the toluene carrier technique . They are all homogeneous, first order processes with a small heterogeneous contribution to the overall rate which produces H2 and CH4.. Rate parameters are collected in Table 25. 

A sample that is too small may not be appropriate because the pyrolysis products cannot be properly detected. Also, the weight of a sample that is too small is difficult to measure with enough accuracy and precision. For materials that are not homogeneous, the smaller the sample, the more difficult it is to obtain a representative sample. Restrictions to the choice of the sample size are also related to the losses due to the possibly incomplete transfer of the pyrolysate to the analytical instrument. This subject will be mentioned further as it relates to the analytical instrumentation attached to the pyrolyser. 

The mechanical problems related to the rapid solid sample introduction or related to the introduction of solid samples with no air leak makes this type of pyrolyser more appropriate for liquid or even gas sample pyrolysis. Also, it being possible to build large furnace pyrolysers, this type is successfully used when larger amounts of sample are necessary to be pyrolysed. This is a common case for the pyrolysis of non-homogeneous samples when a few mg of sample do not represent well the average sample composition. 

The presence of PAA may serve as fingerprints for detection of other functional groups. Unreacted isocyanates used in manufacture of corks may migrate to foodstuffs and hydrolyze there to the corresponding PAA. Thus, cork homogenates in water were extracted with hexane and analyzed by GC-MS for the presence of diamines such as 2,4-diaminotoluene (2e), 2,6-diaminotoluene (2f) and 4,4 -methylenedianiline (5a)65. A combination of pyrolysis with GC-MS was applied to qualitative analysis of azo dyes by detection of aromatic amines in the pyrolysate. This fast detection method was applied in... 

Rapsomanikis and Craig (1991) extracted MeHgCI from fish muscle homogenate into toluene. After back-extraction into a sodium thiosulphate solution, the sample was evaporated to dryness, redissolved in 0.1 M HCI and treated with sodium tetraethyiborate (dissolved in ethanol). After GC separation, the dialkylmercury was pyrolysed in a quartz cell and determined by AAS. The detection limit was 120/[Pg.443]

The homogeneous high temperature pyrolysis of hydrocarbons normally gives similar products in either the presence or absence of limited amounts of oxygen. The effect of oxygen is often to increase the rate of reaction and to vary product ratios. It is the purpose of this paper to examine the oxidative pyrolyses of a series of hydrocarbons under surface free conditions. Achievement of a totally surface free environment has been made possible by the development of the wall-less reactor(lj 2). 

Since the rates of oxidative pyrolyses may be strongly affected by surfaces(2,1,4, a study of the absolute effects of surface is also included. Surface effects are evaluated by comparing reactions done in a homogeneous environment to those with a surface inserted into the reactor. 

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